Casting

Question 1

Freezing range has an inversely proportional relationship with what?

Answer 1

Fluidity

Question 2

What assumptions can be made with a shiny surface finish

Answer 2

A really shiny surface finish is likely not done from casting alone, meaning post processing took place

Question 3

Investment vs shell casting

Answer 3

• Investment has a better surface finish
• Investment has a finer slurry (finer slurry means a thin mixture that can fit smaller crevices but also can make a more compact and smoother surface finish)

Question 4

Lost Wax

Answer 4

• faster cooldown gives a finer grain structure
• finger grain structure usually leads to higher strength and ductility

Question 5

What are the two routes you can go with regarding due casting?

Answer 5

Hot chamber or cold chamber

Question 6

Define cold chamber and give some elements around it

Answer 6

• when the furnace is not directly integrated into the system.
• best suited for metals with higher melting points
- higher melting points has a direct correlation to higher strength and higher viscosity
- due to higher viscosity, higher pressure will be required of the press to push molten metal through
•faster cooling rate ~> smaller grains ~> higher strength

Question 7

Define and describe some elements surrounding hot chamber

Answer 7

• furnace is directly integrated into the system
• best suited for metals with lower melting points
• metals will be quite fluid due to lower melting points
- less pressure will be required for the press
- molten metal will move easier
• slower cooling rate than cold chamber
- slow cooling rate ~> bigger grains ~> lower strength
• slow cooling rate due to the integrated furnace, thus the whole system is at a higher temperature

Question 8

Expendable mold techniques

Answer 8

• Sand
• Ceramic
• Investment
• Plaster
• Evaporative pattern
• Shell

(S~C~I~P~E~S)

Question 9

Permanent mold techniques

Answer 9

• Slush
• Squeeze
• Semisolid
• Pressure
• Die
• Centrifugal

(S~S~S~P~D~C)

Question 10

What is the purpose of a riser in normal gravity casting

Answer 10

They are basically resovoirs for hot metal that helps to supply more metal into the casting during the cooling process to avoid air bubbles or shrinkage in the mold

Question 11

Important considerations in casting

Answer 11

• solidification and cooling of metal in mold
• flow of the molten metal into the mold cavity, and design of gating systems or pathways for molten metal to fill the cavity
• influence of the mold material

Question 12

Is a high or low freezing range desirable?

Answer 12

Low freezing range

Question 13

Low melting temperature alloys are easier to work with

Answer 13

Less input energy to melt metal. Less pressure required, like in the case of hot chamber casting (check those reasons)

Question 14

What are nucleating agents?

Answer 14

Chemical compounds added into hot metal in order to promote nucleation in cooling

Question 15

Aspects of fluidity:

Answer 15

• viscosity
• surface tension
• inclusions (insoluble solid particles entrapped in the fluid)
• solidification pattern of the alloy
• mold design
• mold material and surface characteristic
• degree of superheat (difference between solidification temperature and pouring temperature
• rate of pouring
• heat transfer

Question 16

What kind of objects are used in slush casting?

Answer 16

Hollow, ornamental and decorative objects

Question 17

What is chronov’s rule?

Answer 17

Solidification time is a function of the volume of a casting and its surface area

Question 18

How can dendrites contribute to cooling and be used to strengthen metals for after the cooling process is complete?

Answer 18

Dendrite arms are not particularly strong and can be broken up by agitation or mechanical vibration in the early stages of solidification (also known as semisolid metal forming and rheocasting). This process results in finer grain size, with equipped nondendritic grains distributed more uniformly throughout the casting.

Question 19

What is thixotropic behavior of alloys?

Answer 19

Viscosity decreases when the Liquid Metal is agitated

Question 20

Why are sprue’s tapered?

Answer 20

In free falling liquid, the cross sectional area or the stream decreases as the liquid gains velocity downward. If sprues had a constant cross section, there will be regions where the liquid loses contact with the sprue walls, resulting in ASPIRATION (a process where air is drawn in or entrapped in the liquid).

Question 21

Why are low shrinkage alloys most helpful during the casting process?

Question 22

Why does shrinkage occur?

Answer 22

During cooling, metallic bonds shorten in length when the energy in the structure is lost due to heat losses and cooling

Question 23

Categories of casting defects

Answer 23

A - metallic projections (flash, swell, roughness)
B - cavities (porosity, shrinkage, etc)
C - discontinuities (cracks, hot/cold tearing)
D - defective surface (folds, scars, oxide scale)
E - incomplete casting (misrun)
F - incorrect dimensions and shape
G inclusions

Question 24

Why do hot tears occur?

Answer 24

Casting cannot shrink freely during cooling because of constraints in various parts of the molds and cores. Exothermic compounds may be used (like exothermic padding) to control cooling at critical sections to avoid hot tearing

Question 25

What are the three stages of shrinkage?

Answer 25

1. Contraction of molten metal as it cools prior to its solidification
2. Contraction of the metal during phase change from liquid to solid
3. Contraction of the solidified metal as its temperature drops to ambient temperature

Question 26

Porosity caused by shrinkage can be reduced or eliminated by which means?

Answer 26

• adequate Liquid Metal should be provided to prevent cavities caused by shrinkage
• internal or external chills are an effective means of reducing shrinkage porosity
• with alloys, porosity can be eliminated by HIGH TEMPERATURE GRADIENTS or by INCREASING the COOLING RATE
• subjecting the casting to hot isostatic pressing is another method of reducing porosity

Question 27

What are the function of chills? Both internal and external.

Answer 27

Increases the rate of solidification in critical regions. Internal chills are usually made of the same material as the casting itself and are left in the casting. The problems associated with this is involve improper fusion of the internal chills with the castings themselves.

External chills may be made from the same material as the casting, or may be made of copper, iron or graphite.

Question 28

Hydrogen solubility in aluminium

Answer 28

This is significant because liquid heated aluminium reacts very easily with water vapor in the air and absorbs hydrogen in the process. This results in pinhole porosity in the metal and defects in the casting, this decreasing the tensile strength and ductility of the casting.

Question 29

What’s two ways of getting rid of dissolved gases in a molten metal?

Answer 29

Flushing or purging with an inert gas or melting and pouring the metal in a vacuum.

Question 30

How to get rid of trapped gas if it’s oxygen?

Answer 30

If it’s oxygen, the molten metal can be deoxidised.

Steel can be deoxidised with aluminum, silicon

Copper based alloys can be deoxidised with phosphorous, copper, titanium or zirconium-bearing materials